1. Field of the Invention
The present invention relates to a communication control apparatus for detecting the absence of a control data circulating through a ring communication network, and particularly to a communication control device that can shorten a wait time for issuing a broken control data report signal which indicates the absence of a control data and to quickly carry out a control data recovery process.
2. Description of the Prior Art
A token ring network is a kind of a local area network (LAN) communication system. The token ring network connects a plurality of communication control devices (nodes) in a ring as defined in IEEE Std. 802.5, and circulates a frame having communication data and a control data called a token, which is control a communication of the frame, as a bit stream through the communication control devices in the network.
FIG. 1 shows a block diagram related to a token management function in the communication control devices connected to the token ring network. The communication control device 101 comprises a reception unit 103 for receiving a bit stream from an upstream one of the communication control devices and detecting the token in the data bits; a transmission unit 104 for receiving the communication data from the reception unit 103 and transmitting the same to a downstream one of the communication devices; a timer unit 105 for starting a time counting operation in response to a token detection signal provided by the reception unit 103 and providing a broken token report signal (a time out report signal) if not receiving the next token detection signal within a predetermined time; and a protocol control unit 107 for providing a token transmission signal to the transmission unit 104 when receiving no broken token report signal from the timer unit 105.
Upon receiving the token transmission signal from the protocol control unit 107, the transmission unit 104 generates and transmits the token. The timer unit 105 detects whether or not the token is broken due to noise, etc.
FIG. 2 shows a format of the token.
FIG. 3 is a block diagram showing a conventional timer unit to be disposed in the communication control device of FIG. 1.
In FIG. 3, the timer unit comprises a good token timer 121, which is restarted to count in response to a restart signal, i.e., the token detection signal provided by the reception unit 103 when an enable signal is supplied from the protocol control unit 107. If the good token timer 121 does not receive the next token detection signal from the reception unit 103 within the fixed wait time, the good token timer 121 provides a time out signal, i.e., the broken token report signal and stops counting. If the good token timer 121 receives the next token detection signal from the reception unit 103 within the fixed wait time, the good token timer 121 is restarted as described above.
With the good token timer 121 installed in the timer unit of each communication control device, the number of the communication control devices (nodes) to be connected to the token ring network may be 260 at a maximum. A token holding time of each node is 10 milliseconds (ms), so that if there are 260 nodes, it takes 2.6 sec (10 ms.times.260) at maximum for a token to circulate through the network. Accordingly, the good token timer 121 is usually designed to have the maximum value of 2.6 sec as a fixed wait time.
Namely, if the token is not again detected 2.6 sec after detecting the token for the first time, the good token timer 121 provides the broken token report signal indicating that the token was broken by noise, etc., and that a recovery process such as sending again the token must be carried out.
FIG. 4 shows a token ring network 125 including four nodes (communication control devices) A to D each having the good token timer 121 of FIG. 3. This network circulates a token in a counterclockwise direction.
In each of the nodes A to D, the good token timer 121 is restarted to count the fixed wait time of 2.6 sec whenever receiving the token. If the good token timer 121 of a certain one of the nodes A to D again receives the token within the fixed wait time, the normal data communication is carried out.
If the circulating token is broken due to noise, etc., between, for example, the nodes A and B, the good token timer 123 of the node B causes a time out at first because the timer 123 of the node B has been started earliest. The node B, therefore, provides the broken token report signal.
As explained before, a maximum token holding time of each node for holding a token for transmitting communication data is 10 ms. In the token ring network 125 with the four nodes A to D of FIG. 5, a maximum time for circulating the token through the network 125 is 40 ms (10 ms.times.4 nodes), so that the 40 ms is sufficient as the wait time set in the good token timer 121.
The wait time of the good token timer 121, however, is fixed to 2.6 sec in consideration of the maximum node number of 260 possibly connected to the token ring network. As a result, although the 40 ms is sufficient as the wait time for determining whether or not the token is broken in the small token ring network with the four nodes shown in FIG. 5, each node of the network of FIG. 5 must have a redundancy of 2.56 sec (2.6 sec -40 ms). For this redundancy, the broken token report signal is delayed, and data transmission is disabled. This means that, if the token ring has a data transfer rate of 4 Mbps, data of about 10 megabits are not transmitted during the redundancy of 2.56 sec, to cause a lowering of communication efficiency.
When no data is transmitted from any one of the four nodes, the token is circulated at high speed through the network 125 because the token is not kept in each node. In this case, the broken token report signal will never be provided by any one of the nodes before the expiration of the fixed wait time of 2.6 sec, and during this period, no communication will be made, thereby aggravating the problem.